The following relates in general, to the field of nuclear power reactor arts, nuclear reaction control apparatus arts, control rod assembly arts, and related arts. More particularly, the following is directed to control rod guide frames and manufacturing methods thereof.
In nuclear power plants, a nuclear reactor core comprises a fissile material having size and composition selected to support a desired nuclear fission chain reaction. To moderate the reaction, a neutron absorbing medium may be provided, such as light water (H2O) in the case of light water reactors, or heavy water (D2O) in the case of heavy water reactors. The reaction may be controlled or stopped by inserting “control rods” comprising a neutron-absorbing material into aligned passages within the reactor core. When inserted, the control rods absorb neutrons so as to slow or stop the chain reaction. The control rods are operated by control rod drive mechanisms (CRDMs). In so-called “gray” control rods, the insertion of the control rods is continuously adjustable so as to provide continuously adjustable reaction rate control. In so-called “shutdown” control rods, the insertion is either fully in or fully out. During normal operation the shutdown rods are fully retracted from the reactor core; during a SCRAM, the shutdown rods are rapidly fully inserted so as to rapidly stop the chain reaction. Control rods can also be designed to perform both gray rod and shutdown rod functions. Typically, a number of control rods are connected with a single CRDM by an assembly including a connecting rod coupled with the CRDM and terminating in a “spider” or other coupling element that supports the plural control rods. In such an assembly, the CRDM moves the plural control rods, along with the spider and the connecting rod, together as a unit.
The control rod guide frames of a typical PWR reactor are housed in the reactor internals. The control rod guide frames provide an alignment feature for the control rod assembly between the nuclear reactor core (i.e. fuel) and the CRDMs external to the pressure vessel (or, in some embodiments, internal to the pressure vessel and located above the reactor core). The guide frames provide guidance for the control rod assembly when being withdrawn from the fuel for both power shaping (i.e. gray) rods and shutdown rods. When the control rods are partially or wholly withdrawn from the reactor core, they are guided by the control rod guide frame so as to ensure that the control rods remain in precise alignment with the aligned passages (i.e. guide tubes) within the reactor core. The guide frame provides lateral support for the control rods within the control rod assembly. Some guide frames provide continuous support of the control rods while others provide periodic support of the control rods through the use of spaced-apart guiding plates.
In one typical guide frame configuration, a plurality of spaced apart guide plates is secured together by a frame, e.g. tie rods. In operation, the control rods are guided as they pass through openings in the guide plates. Such a guide frame design has advantages including low weight and material cost, and limited impedance of primary coolant flow due to the largely open design. The use of guide plates to define the control rod guiding surfaces also provides a convenient planar form for the precision metalwork.
Shargots et al., U.S. Pat. Pub. No. 2012/0099691 A1 discloses a control rod guide frame comprising a self-supporting stack one or more columnar elements defining a central passage. The columnar elements may include mating features that mate at abutments between adjacent columnar elements of the stack. The control rod guide frame is suitably used in conjunction with a CRDM operatively connected with at least one control rod, and a nuclear reactor core, in which the CRDM moves the at least one control rod into and out of the nuclear reactor core under guidance of the control rod guide frame. In another embodiment, a control rod guide frame comprises a stack of one or more columnar elements defining a central passage having a constant cross-section as a function of position along the central passage. In another embodiment, a control rod guide frame comprises an extruded columnar element providing continuous control rod guidance. Shargots et al., U.S. Pat. Pub. No. 2012/0099691 A1 is hereby incorporated into this patent application by reference in the entirety.
Being able to provide continuous support of the control rods, e.g. as in embodiments of U.S. Pat. Pub. No. 2012/0099691 A1, is advantageous because there is a reduced risk of control rod buckling during rod insertion into the fuel, either by SCRAM or a controlled shutdown. In addition, continuously supporting the control rods helps reduce flow induced excitation of the control rods in turn helping reduce wear.
In guide frames constructed from a series of plates spaced a distance apart from each other with segmented tubes inserted between the plates and then welded into place at every location, the manufacturing process is labor intensive, requires a significant number of welds, and is susceptible to welding induced distortions.